This invention relates in general to imaging and in particular to electrography, including xerography, using liquid developers.
The use of electrostatic charge patterns to form images is well-known in the art. Probably the more common process in which electrostatic charge patterns are used to form images is xerography. In the art of xerography, exemplified by Carlson, U.S. Pat. No. 2,297,691, it is usual to employ the simultaneous application of electric field and a pattern of activating radiation on a photoconductive insulating member to form an electrostatic charge pattern otherwise known as an electrostatic latent image. This electrostatic latent image is then capable of being utilized such as, for example, by the deposition of electrostatic material thereon to form a visible image. Usually, the order of procedure is to sensitize the xerographic plate by applying a uniform charge to the surface to the photoconductive member after which exposure is made. Sensitization may be accomplished by any of various means, such as, for example, frictional means as disclosed in the Carlson patent or ion charging means as shown in Carlson U.S. Pat. No. 2,588,699.
Other techniques for forming electrostatic charge patterns are known. For example, in U.S. Pat. No. 3,518,081 to Bickmore and Goffe an imagewise modification is made to a surface to alter imagewise capability of the surface to accept charge. By triboelectrically charging such a surface, a charge pattern is formed which may be developed by contacting the surface with colored electroscopic marking particles which are attracted to the charge pattern.
Another broad general branch of electrostatography is generally referred to as electrography and it is considered distinct from the xerographic branch in that it does not employ a photoconductor and light exposure to control the formation of its latent electrostatic charge pattern. Electrography as it is generally known today may be divided into two broad sectors which are, xeroprinting and TESI recording. Xeroprinting may be described as the electrostatic analog of ordinary printing. This process, which is more fully described in U.S. Pat. No. 2,576,047 to Schaffert, employs a xeroprinting plate made up of a pattern of insulating material on a conductive backing so that when the xeroprinting plate is charged, as with a corona discharge electrode, an electrostatic charge pattern is retained only on the patterned insulating sections of the plate. Image development is by the same techniques employed in xerography.
The common feature of all of these electrostatographic systems is that they employ the lines of force from the electric field of a latent electrostatic image to control the deposition of colored finely divided marking particles known as toner, thus forming a visible image corresponding to the charge pattern. The colored finely divided marking particles may be in dry powder form or may be suspended in an insulating liquid. The use of insulating liquids containing developer particles is well-known to the art. See, for example, U.S. Pat. No. 3.053,688 to Greig.
The above tehniques are useful for forming hard copy or for displays. Where it is desired to reuse the member on which the image is formed, it is necessary to remove the image, clean the member, and recharge the member, either imagewise, or uniformly with subsequent imagewise dissipation of the charge.
In addition to the use of marking particles to make charge patterns visible, other methods are known. In one such method described in U.S. Pat. No. 3,196,010 to Goffe et al. a thin, from about 80 to about 125 millimicrons, layer of an insulating "developer" liquid is formed over the charge pattern. The electrostatic image creates interference or light scattering patterns in the liquid which are visible. The liquid may also contain a wax or dissolved material which will form a more permanent solid image as taught in U.S. Pat. No. 3,196,010 to W. L. Goffe et al. issued July 20, 1965, the disclosure of which is incorporated herein by reference.
A further technique for making electrostatic latent images visible is by using liquid crystal materials. The electrostatic field causes phase changes or changes in optical polarization which appear as differences in color or opacity. The use of liquid crystal materials to develop electric fields is shown in U.S. Pat. No. 3,642,384, issued Feb. 15, 1972, and U.S. Pat. No. 3,707,322, issued Dec. 26, 1972, which patents are assigned to the assignee of this application and which disclosures are incorporated herein by reference.
The techniques described in the last two paragraphs above are particularly useful for image display purposes. Again, in order to reuse the member on which the image is formed, it is usually necessary to remove the image, clean the member, and form a new charge pattern on the member. Normally, the above processes are carried out using ion charging means such as a corona discharge or by contacting the free surface of the developing liquid with a second electrode and applying a potential difference between the photoconductive member and the second electrode. These techniques, although successful, require expensive and bulky equipment, and high potentials. Further, usually several processing steps are required to form a charge pattern, develop the image, remove the image, clean the member on which the image is formed and to reform the charge pattern.